Kris Jenner was her usual glamorous, friendly self during a grocery run at Gelson’s in Beverly Hills on Monday. But her happy smile was hiding sordid secret: RadarOnline.com has exclusively learned that Jenner is desperately trying to untangle herself from a sleazy sex tape extortion plot — and has even gotten law enforcement involved!

According to a source close to the Kardashian-Jenner family, Jenner was approached over a week ago by an unknown man claiming to be in possession of a sex tape starring the Keeping Up with the Kardashians momager.

“At first she thought it was a joke, because she knew it couldn’t be real,” the source told Radar. “Obviously she would never allow herself to be taped like that! Initially, she just ignored it.”

“He called Kris more than one hundred times in the last seven days, from more than 100 different phone numbers,” the source revealed. “He uses scrambled cell numbers. He called more than 25 times just yesterday (Sunday) alone!”

“The man has been demanding money from Kris. If he’s not paid, he claims, he’ll release the tape.”

Even more terrifying, 58-year-old Jenner has reason to fear she could be being watched.

“She thinks he could be following her,” the source confirmed. “She went out to eat and then got a text from him saying ‘Are you enjoying the restaurant? This person knew where she was.”

As a result, the insider revealed, “The family has drastically stepped up their security and Kris had no choice but to turn all the information over to law enforcement and to [the family’s private investigator] Gavin DeBecker.”